The main property of a Faraday cage is formation of an isolating barrier against coupling electromagnetic waves between the inside and the outside of the Faraday cage. This property is used either to prevent an electromagnetic signal that is emitted inside the Faraday cage from being picked up outside it, as is particularly true when it is desired to protect secret messages transmitted by telecommunications means; or elst to protect the inside of the cage against external electromagnetic influences, for example to protect apparatus which is sensitive to electromagnetic waves from a magnetic storm as may be triggered by a nuclear attack.
The main problem with using Faraday cages is feeding electricity to apparatus disposed inside the cage. If an electricity power cable is merely passed through the wall of the Faraday cage, even if the opening through the wall has dimensions appropriate for forming a waveguide, then the electric cable is subjected to electromagnetic radiation on either side of the wall, and it transmits the radiation by conduction to the other side.
In order to avoid transmitting electromagnetic waves via the electricity power supply cable, Faraday cages have already been fed with electricity via filters disposed at the wall of the cage, with the filter housings being connected to the cage wall. However, such filters which are generally constituted by capacitors and inductors and are matched only to certain frequencies of electromagnetic waves, thereby giving rise to a limited filtering effect. Further, leakage currents from these filters, which may be quite high, are to be found in the grounding of the cage and may thus be picked up from outside the cage and decoded in order to extract information about the electromagnetic waves which are emitted inside the cage.
Preferred embodiments of the present invention provide a device for feeding electricity into a Faraday cage without transmitting any electromagnetic phenomena between the inside and the outside of the Faraday cage.